1. Field of the Invention
The invention relates to a process for the recovery of unreacted monomers which occur in the residual gas stream of the vinyl ester-ethylene copolymer preparation.
2. Description of the Related Art
Polymers based on vinyl esters and ethylene are frequently used as aqueous dispersions or polymer powders redispersible in water in all kinds of applications, for example as coating materials or adhesives for a very wide range of substrates.
In industrial-scale polymerization, high conversions are state of the art. Thus, the polymers are usually polymerized to a residual monomer content of less than 0.1% by weight, preferably even less than 0.05% by weight, and, in the case of vinyl chloride, of less than 0.01% by weight. However, these high conversions relate only to the monomers which are liquid under polymerization conditions. Ethylene as a monomer which is gaseous under polymerization conditions does not comply with these laws. Firstly, it polymerizes more slowly than, for example, vinyl acetate and secondly it is present to a large proportion in the gas phase and cannot be included under the usual conditions of emulsion polymerization.
From economic points of view, the industrial polymerization should be complete in periods which are as short as possible, which, however inevitably means that the ethylene used cannot be completely consumed. Usually, the polymerization is stopped at a residual ethylene content of less than 5% by weight, preferably less than 2% by weight, and the reaction batch is let down to atmospheric pressure. The flash process includes the transfer of the reaction mixture comprising polymer dispersion and residual gas from a pressure reactor to a reactor at atmospheric pressure, the so-called flash vessel, with removal of the residual ethylene. The resulting residual gas contains mainly ethylene, vinyl acetate, nitrogen and steam.
A conventional method from the prior art is the utilization of the resulting residual gas for energy through combustion. In view of increasing raw material costs, the physical recovery of the individual components and monomers has substantial advantages from the economic point of view.
The prior art discloses various processes for residual ethylene recovery. Patent WO 01/00559 A1 describes a process for recovering ethylene from the ethylene-containing inert gas stream obtained in vinyl acetate production. For this purpose, the ethylene-containing inert gas is taken up in vinyl acetate, and the ethylene is liberated in a vacuum vessel by flashing and is then recompressed for re-use. A process in which the ethylene-containing inert gas stream is taken up in acetic acid and brought into contact with ethylene-containing residual gas in a stripping column and the ethylene is recovered at the top of the column and re-used in the vinyl acetate synthesis is furthermore described.
The patent EP 0 127 253 A1 relates to the residual monomer removal in the gas-phase polymerization of copolymers of ethylene and higher olefins. For this purpose, the solid copolymer is freed from the higher olefins at reduced pressure and then treated with inert gas-free reactor gas, which is finally recycled to the polymerization.
All methods known to date from the prior art have the disadvantage that they do not optimally utilize the resulting residual gas.